1 (define (test-case actual expected)
2   (newline)
3   (display "Actual:   ")
4   (display actual)
5   (newline)
6   (display "Expected: ")
7   (display expected)
8   (newline))
9 
10 (define (wrong-count-pairs x)
11   (if (not (pair? x))
12       0
13       (+ (count-pairs (car x))
14          (count-pairs (cdr x))
15          1)))
16 
17 (define (last-pair x)
18   (if (null? (cdr x))
19       x
20       (last-pair (cdr x))))
21 
22 (define (make-cycle x)
23   (set-cdr! (last-pair x) x)
24   x)
25 
26 (define three '(a b c))
27 (define a-pair (cons '() '()))
28 (define b-pair (cons a-pair a-pair))
29 (define four (cons 'a b-pair))
30 (define seven (cons b-pair b-pair))
31 (define circular (make-cycle '(a b c)))
32 
33 ;; (test-case (wrong-count-pairs three) 3)
34 ;; (test-case (wrong-count-pairs four) 4)
35 ;; (test-case (wrong-count-pairs seven) 7)
36 ;; (test-case (wrong-count-pairs circular) 'infinite-loop)
37 
38 ;; Devise a correct version of the count-pairs procedure of exercise 3.16 that returns the number of distinct pairs in any structure. (Hint: Traverse the structure, maintaining an auxiliary data structure that is used to keep track of which pairs have already been counted.) 
39 
40 (define (count-pairs x)
41   (let ((traversed-pairs '()))
42     (define (not-traversed x)
43       (cond ((not (pair? x)) 0)
44 	    ((memq x traversed-pairs) 0)
45 	    (else (set! traversed-pairs (cons x traversed-pairs))
46 		  (+ (not-traversed (car x))
47 		     (not-traversed (cdr x))
48 		     1))))
49     (not-traversed x)))
50 
51 (test-case (count-pairs three) 3)
52 (test-case (count-pairs four) 3)
53 (test-case (count-pairs seven) 3)
54 (test-case (count-pairs circular) 3)
55